Polychlorinated biphenyls (PCB compounds) were first discovered to be environmental pollutants in 1966. They have been round throughout the world in water, solid sediments, and bird and fish tissue. There are some 209 different PCB compounds available, made by substituting from 1 to 10 chlorine atoms onto a biphenyl aromatic structure. PCB compounds have very high chemical, thermal and biological stability, and a low, water solubility and vapour pressure. While these useful properties contributed to their widespread use, those same properties allowed these compounds to be accumulated in the environment.
The manufacture of PCB compounds was discontinued in the United States in 1979, although these compounds continue to enter the environment from discarded electrical equipment, etc. PCB concentrations of 1-2 ppm are normally the desired maxima, and levels of 10-50 ppm in agricultural soils, clays or marine sediments are considered hazardous. The dense and hydrophobic nature of PCB compounds ensures that their accumulation in river sediment is commonplace, leading to bioaccumulation in bottom dwellers and fish thus leading to entry into the human food chain. PCB compounds can reduce human disease resistance, and increase the incidence of rashes, liver ailments and headaches. Similarly, pesticides can have serious health effects on humans and animals.
Numerous investigations of ways to degrade PCB compounds and pesticides have been carried out. At present there are no widely accepted methods for the large scale remediation of water or soils contaminated with PCB compounds or pesticides. The decomposition of PCB and organochloride compounds can be effected by high temperature incineration at a typical temperature of 1300° C. but the gaseous products must be quenched quickly to avoid the reformation of the PCB or the formation of undesirable side reaction products such as dioxins at 800 -900° C. Such a process is complicated and with variable or uncertain outcomes. Biodegradation with microorganisms and chemical treatment are methods which require lengthy treatment periods. Photocatalytic (UV) degration of contaminated soil-water systems has also been tried but is also slow.
Ultrasound is known in the art for inducing chemical reaction processes in liquids, a field known as sonochemistry. The propagation of ultrasonic waves in a liquid generates cavitation bubbles. These bubbles implode and produce micro-regions of extreme conditions. Estimated temperatures within these micro-regions range from 2000-5000K in aqueous solution. In U.S. Pat. No. 5,498,431 a process is described for decontaminating particulate surfaces by the use of ultrasound to firstly release mycotoxins from the particulates into an aqueous liquid followed by a chemical reaction breakdown of the contaminants by ultrasound when in the liquid. The cavitation from the ultrasound leads to a sonochemical breakdown reaction of the mycotoxin contaminants when in the aqueous liquid. In WO96/20784 a method of chemical reaction catalysis in a liquid is described which is facilitated by ultrasonic cavitation. The cavitation is aided by the presence of solid particles as a surface for ‘seeding’ the cavitation bubbles prior to their separation from the solid particles whereupon the bubbles cavitate (implode) in the liquid medium.
Ultrasound has been used to decompose PCB compounds that are dissolved in an aqueous solution. However, because of their low solubility, the concentration of PCB compounds in aqueous solution is very low when compared with that found adsorbed onto solids, river sediment and the like, so that such an aqueous treatment technique is largely ineffective.
It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.